Conventionally, polymers that contain a high hydrogen bonding group with high hydrophilicity in their molecules, represented by poly(meth)acrylic acid and polyvinyl alcohol are used as gas barrier polymers. However, while exhibiting very excellent gas barrier properties against oxygen and the like under dry conditions, films made up of only any one of these polymers pose, under a high moisture conditions, problems that their gas barrier properties against oxygen and the like are greatly decreased due to their hydrophilicity, and that the films exhibit inferior resistance against humidity and hot water.
To solve these problems, for example, International Publication No. WO03/091317 (Document 1) discloses a film that uses a polycarboxylic acid-based polymer and a multivalent metal compound as starting materials and has a peak ratio (A1560/A1700) of the infrared absorption spectra of 0.25 or higher. The specification describes a technique involving forming a laminate in which a layer made of a polycarboxylic acid-based polymer and a layer made of a multivalent metal compound are adjacent to each other, and then crosslinking the polycarboxylic acid-based polymer with the multivalent metal.
In addition, as a technique related to the above-described technique, for example, Japanese Unexamined Patent Application Publication No. 2005-125693 (Document 2) discloses a gas barrier laminated film wherein a gas barrier layer made up of a crosslinking substance of a polycarboxylic acid and a multivalent metal compound is formed on at least one side of a film substrate. The specification describes that a substance in which a polycarboxylic acid is partially neutralized with an alkali such as sodium hydroxide, lithium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, aluminum hydroxide or ammonia is used as a polycarboxylic acid.
However, it has been found that the gas barrier films described in the above documents and the like, and a package using these films have a problem that their performances are decreased depending on manufacturing processes and applications. That is, whitening may occur or gas barrier properties or transparency may be damaged, in the case of exposure to cold water, for example, the case where an end face part of the film or the package comes into contact with cold water, or is contaminated with attachment of cold water or the case where the package is washed prior to heat sterilization or subjected to immersion treatment, in a process of manufacturing the film or the package.